1. Field of the Invention
This invention relates to an evaporative emission control system for an internal combustion engine for automotive vehicles, for controlling emission of evaporative fuel generated in a fuel tank of the engine, and more particularly to an evaporative emission control system which is adapted to collect and store evaporative fuel generated from the fuel tank in a charcoal canister, and controls emission of the stored evaporative fuel by purging it with purging air to the engine.
2. Prior Art
In automotive vehicles, evaporative fuel is generated in a fuel tank thereof, particularly when they are standing at a high temperature. The evaporative fuel, which is composed of hydrocarbons, will cause air pollution if it is freely emitted into the air. Therefore, it is necessary to prevent it from being emitted into the atmosphere. For this purpose, a charcoal canister is generally connected to the fuel tank for collecting and storing therein evaporative fuel generated in the fuel tank. The charcoal canister is in the form of a container containing an adsorbent, such as activated carbon, for adsorbing evaporative fuel, with a port opening into the atmosphere. Usually, the charcoal canister is connected to the intake system of the engine via a purging passage.
When the engine is in operation, outside air is drawn into the charcoal canister via the port of the charcoal canister, due to negative pressure generated in the intake system of the engine. Accordingly, the evaporative fuel adsorbed by the adsorbent in the charcoal canister is carried off the adsorbent by the outside air drawn in, and purged via the purging passage into the intake system. The evaporative fuel and the outside air, i.e. purging air are drawn together in the form of a mixture via the intake system into the combustion chambers of the engine, where the mixture is burned.
By thus purging the evaporative fuel adsorbed by the adsorbent in the charcoal canister when the engine is in operation, the fuel-adsorbing efficiency of the charcoal canister is restored for use when the engine is in stoppage.
The purging air thus introduced via the charcoal canister into the intake system at the start of the engine contains fuel having a relatively high concentration. Therefore, if a large amount of purging air is allowed to be introduced into the intake system when the engine is not warmed up at the start thereof or when the engine in a low engine rotational speed-low load condition in which the total amount of intake air is small, the engine may become unstable.
Therefore, in general, the amount of purging air and evaporative fuel purged is controlled depending on operating conditions of the engine.
For example, a purging port is provided at a terminal end of the purging passage which opens into the intake system and via which purging air is introduced into the intake system as disclosed, for example, by Japanese Provisional Patent Publication (Kokai) No. 59-58143. The purging port is located at such a so-called Pc port location that the purging port is positioned upstream of the throttle valve when the latter is fully closed and positioned downstream of same when it is opened to a medium or larger degree. The provision of the purging port at the Pc port location thus causes the purging port to be positioned upstream of the throttle valve when the engine is in a low load condition, so that the pressure of intake air in the vicinity of the purging port is maintained at atmospheric pressure. Accordingly, the outside air is not drawn into the charcoal canister, inhibiting the purging of evaporative fuel. On the other hand, when the engine is in a high load condition in which the throttle valve is fully opened, the pressure of intake air in the vicinity of the purging port is approximately equal to atmospheric pressure, which prevents the purging of evaporative fuel. When the throttle valve is half opened, assuming a medium opening degree, negative pressure created in the intake pipe is applied to the purging passage via the purging port, which causes the outside air to be drawn into the purging passage via the charcoal canister, effecting purging of evaporative fuel.
In short, the purging port provided in the Pc port location has the function of controlling the amount of evaporative fuel purged, in dependence on load on the engine.
Further, an evaporative emission control system has been proposed by Japanese Utility Model Publication (Kokoku) No. 60-27819, in which the purging of evaporative fuel is not carried out when the engine is not warmed up, whereas the purging air is introduced into the intake system of the engine when the engine is in a predetermined temperature range, and when the temperature of the engine is higher than the upper limit of the predetermined range, the purging air is introduced into the exhaust system of the engine. According to this evaporative emission control system, an intake purging passage and an exhaust purging passage are provided for connecting between a charcoal canister, and the intake system and the exhaust system of the engine, respectively, and one of the purging passages is selected for use by means of a selector valve which operates in response to the temperature of engine coolant.
According to the proposed evaporative emission control system, the purging air is processed in the exhaust system when the engine is at a high temperature in a normal operating condition, which prevents the air-fuel ratio of a mixture supplied to the engine from being adversely affected.
On the other hand, in recent years, regulations on emission of evaporative fuel as unburned gas into the air have become more strict, and accordingly a charcoal canister for adsorbing or storing the evaporative fuel is generally increased in size, which has increased capacity of absorbing and storing evaporative fuel therein. Therefore, it is required to increase the amount of evaporative fuel purged. However, if the amount of evaporative fuel purged or purging air is controlled by the purging port arranged at the Pc port location as in conventional systems, the purging of evaporative fuel is not effected when the engine is in a low load condition, i.e. when the engine is idling or decelerating. Therefore, the amount of evaporative fuel purged is limited, which makes it impossible to fully utilize the adsorbing capability of the charcoal canister. On the other hand, according to the proposed system adapted to introduce the purging air into the exhaust system when the engine is at a high temperature, the purging air is always processed in the exhaust system after the engine has been warmed up, which results in a waste of evaporative fuel contained in the purging air, greatly increasing the fuel consumption. Moreover, when the pressure of exhaust gases is high, the purging air cannot be introduced into the exhaust system, which imposes a substantial limit on an engine operating condition region in which the purging of evaporative fuel is permitted, and hence makes it difficult to increase the amount of evaporative fuel purged.